Tropical Storm LOWELL

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TROPICAL STORM LOWELL DISCUSSION NUMBER   8
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   EP132008
200 PM PDT MON SEP 08 2008
 
LOWELL CONTINUES TO EXPERIENCE ABOUT 20 KT OF EASTERLY VERTICAL WIND
SHEAR...WHICH IS CAUSING THE LOW-LEVEL CENTER TO BE PARTLY EXPOSED
AT THE NORTHEASTERN EDGE OF A PULSING AND PERSISTENT CONVECTIVE
MASS.  SATELLITE INTENSITY ESTIMATES ARE 45 KT FROM SAB AND 35 KT
FROM TAFB....WHILE A QUIKSCAT OVERPASS THIS MORNING SHOWED ONE 45
KT WIND.  THE INITIAL INTENSITY IS HELD AT 45 KT...AND RECENTLY
RECEIVED LOW-RESOLUTION ASCAT DATA SUGGESTS THIS MAY BE GENEROUS.
 
THE INITIAL MOTION IS NOW 315/10.  THERE IS LITTLE CHANGE TO THE
EARLIER FORECAST PHILOSOPHY.  LOWELL IS CURRENTLY BEING STEERED BY
A MID-LEVEL RIDGE OVER NORTHERN MEXICO.  ALL OF THE DYNAMICAL
MODELS FORECAST A MID/UPPER-LEVEL TROUGH...CURRENTLY SEEN
DEVELOPING IN WATER VAPOR IMAGERY...TO AMPLIFY SOUTHWARD ALONG THE
WEST COAST OF THE UNITED STATES DURING THE NEXT COUPLE OF
DAYS.  THIS SHOULD TURN LOWELL TOWARD THE NORTH AND EVENTUALLY
NORTHEAST.  WHILE THE GUIDANCE AGREES ON THIS SCENARIO...THERE IS
MAJOR DISAGREEMENT ON HOW FAST LOWELL WILL MOVE AFTER RECURVATURE.
THE HWRF...LBAR...AND BAMD MOVE THE STORM VERY RAPIDLY...WITH 120 HR
POSITIONS OVER THE CENTRAL UNITED STATES.  ON THE OTHER HAND...THE
GFS AND BAMS KEEP THE LOW-LEVEL CENTER WEST AND SOUTHWEST OF BAJA
CALIFORNIA THROUGH 120 HR.  THE DIFFERENCE BETWEEN THE BAMS AND
BAMD MODELS IS STILL STRIKING AND SUGGESTS THE POSSIBILITY THAT
LOWELL COULD SHEAR APART AFTER 48 HR.  THE NEW FORECAST TRACK IS A
SLOW-MOTION COMPROMISE BETWEEN THESE EXTREMES...SHOWING LOWELL
SLOWLY RECURVING TO THE NORTHEAST AND REACHING THE SOUTHERN BAJA
PENINSULA IN 72-96 HR.  THE FORECAST TRACK IS A LITTLE SLOWER THAN
THE OVERALL MODEL CONSENSUS AND IN BEST AGREEMENT WITH THE GUNA
CONSENSUS MODEL AND THE FLORIDA STATE SUPERENSEMBLE.
 
AS BEFORE...THE DYNAMICAL MODELS FORECAST THE CURRENT SHEAR TO
DIMINISH IN ABOUT 24 HOUR.  HOWEVER...LOWELL IS FORECAST TO MOVE
TOWARD COOLER SEA SURFACE TEMPERATURES DURING THE NEXT 48 HR...AND
THIS REDUCES THE CHANCE OF INTENSIFICATION WHEN THE SHEAR
DECREASES.  AFTER RECURVATURE...LOWELL MAY FIND WARMER WATER...BUT
IS ALSO EXPECTED TO ENCOUNTER INCREASING SOUTHERLY AND
SOUTHWESTERLY SHEAR.  THE INTENSITY GUIDANCE IS IN GOOD AGREEMENT
ON A SLOW WEAKENING DURING THE FORECAST PERIOD.  THE INTENSITY
FORECAST AGAIN CALLS FOR A GRADUAL WEAKENING ...WITH THE
INTENSITIES HELD A LITTLE ABOVE ALL THE GUIDANCE.  AN ALTERNATIVE
SCENARIO WOULD BE FOR LOWELL TO SHEAR APART AFTER 48 HR...WHICH
WOULD LEAD TO DISSIPATION BEFORE THE END OF THE FORECAST PERIOD.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      08/2100Z 19.8N 112.1W    45 KT
 12HR VT     09/0600Z 20.4N 112.7W    45 KT
 24HR VT     09/1800Z 21.2N 113.4W    40 KT
 36HR VT     10/0600Z 22.1N 113.5W    40 KT
 48HR VT     10/1800Z 23.1N 113.1W    35 KT
 72HR VT     11/1800Z 24.5N 112.0W    35 KT...NEAR BAJA COAST
 96HR VT     12/1800Z 25.5N 111.0W    30 KT...INLAND
120HR VT     13/1800Z 26.5N 110.0W    30 KT...OVER WATER
 
$$
FORECASTER BEVEN
 
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